CSCE 330 Programming Language Structures

1. CSCE 330Programming Language Structures Fall 2004 Marco Valtorta mgv@cse.sc.edu

2. Textbooks • Ghezzi and Jazayeri • The main textbook • History and general concepts • Syntax and semantics • Imperative languages • Functional languages • Declarative languages • Ullman • In-depth coverage of the functional language ML-97

3. Disclaimer • The slides are based on the textbooks and other sources, including several other fine textbooks for the Programming Language (PL) Concepts course • The PL Concepts course covers topics PL1 through PL11 in Computing Curricula 2001 • One or more PL Concepts course is almost universally a part of a Computer Science curriculum

4. Why Study PL Concepts? • Increased capacity to express ideas • Improved background for choosing appropriate languages • Increased ability to learn new languages • Better understanding of the significance of implementation • Increased ability to design new languages • Background for compiler writing • Overall advancement of computing

5. Software Development Process • Three models of the Software Development process: • Waterfall Model • Spiral Model • RUDE • Run, Understand, Debug, and Edit • Different languages provide different degrees of support for the three models

6. The Waterfall Model • Requirements analysis and specification • Software design and specification • Implementation (coding) • Certification: • Verification: “Are we building the product right?” • Validation: “Are we building the right product?” • Module testing • Integration testing • Quality assurance • Maintenance and refinement

7. PLs as Components of a Software Development Environment • Goal: software productivity • Need: support for all phases of SD • Computer-aided tools (“Software Tools”) • Text and program editors, compilers, linkers, libraries, formatters, pre-processors • E.g., Unix (shell, pipe, redirection) • Software development environments • E.g., Interlisp, JBuilder • Intermediate approach: • Emacs (customizable editor to lightweight SDE)

8. Influences on PL Design • Software design methodology (“People”) • Need to reduce the cost of software development • Computer architecture (“Machines”) • Efficiency in execution • A continuing tension • The machines are winning

9. Software Design Methodology and PLs • Example of convergence of software design methodology and PLs: • Separation of concerns (a cognitive principle) • Divide and conquer (an algorithm design technique) • Information hiding (a software development method) • Data abstraction facilities, embodied in PL constructs such as: • SIMULA 67 class, Modula 2 module, Ada package, Smalltalk class, CLU cluster, C++ class, Java class

10. Abstraction • Abstraction is the process of identifying the important qualities or properties of a phenomenon being modeled • Programming languages are abstractions from the underlying physical processor: they implement “virtual machines” • Programming languages are also the tools with which the programmer can implement the abstract models • Symbolic naming per se is a powerful abstracting mechanism: the programmer is freed from concerns of a bookkeeping nature

11. Data Abstraction • In early languages, fixed sets of data abstractions, application-type specific (FORTRAN, COBOL, ALGOL 60), or generic (PL/1) • In ALGOL 68, Pascal, and SIMULA 67 Programmer can define new abstractions • Procedures (concrete operations) related to data types: the SIMULA 67 class • In Abstract Data Types (ADTs), • representation is associated to concrete operations • the representation of the new type is hidden from the units that use the new type • Protecting the representation from attempt to manipulating it directly allows for ease of modification.

12. Control Abstraction • Control refers to the order in which statements or groups of statements (program units) are executed • From sequencing and branching (jump, jumpt) to structured control statements (if…then…else, while) • Subprograms and unnamed blocks • methods are subprograms with an implicit argument (this) • unnamed blocks cannot be called • Exception handling

13. Non-sequential Execution • Coroutines • allow interleaved (not parallel!) execution • can resume each other • local data for each coroutine is not lost • Concurrent units are executed in parallel • allow truly parallel execution • motivated by Operating Systems concerns, but becoming more common in other applications • require specialized synchronization statements • Coroutines impose a total order on actions when a partial order would suffice

14. Computer Architecture and PLs • Von Neumann architecture • a memory with data and instructions, a control unit, and a CPU • fetch-decode-execute cycle • the Von Neumann bottleneck • Von Neumann architecture influenced early programming languages • sequential step-by-step execution • the assignment statement • variables as named memory locations • iteration as the mode of repetition

15. Other Computer Architectures • Harvard • separate data and program memories • Functional architectures • Symbolics, Lambda machine, Mago’s reduction machine • Logic architectures • Fifth generation computer project (1982-1992) and the PIM • Overall, alternate computer architectures have failed commercially • von Neumann machines get faster too quickly!

16. Language Design Goals • Reliability • writability • readability • simplicity • safety • robustness • Maintainability • factoring • locality • Efficiency • execution efficiency • referential transparency and optimization • optimizability: “the preoccupation with optimization should be removed from the early stages of programming… a series of [correctness-preserving and] efficiency-improving transformations should be supported by the language” [Ghezzi and Jazayeri] • software development process efficiency • effectiveness in the production of software

17. Language Translation • A source program in some source language is translated into an object program in some target language • An assembler translates from assembly language to machine language • A compiler translates from a high-level language into a low-level language • the compiler is written in its implementation language • An interpreter is a program accepts a source program and runs it immediately • An interpretive compiler translates a source program into an intermediate language, and the resulting object program is then executed by an interpreter

18. Example of Language Translators • Compilers for Fortran, COBOL, C • Interpretive compilers for Pascal (P-Code) and Java (Java Virtual Machine) • Interpreters for APL and (early) LISP

19. Plankalkül (Konrad Zuse, 1943-1945) FORTRAN (John Backus, 1956) LISP (John McCarthy, 1960) ALGOL 60 (Transatlantic Committee, 1960) COBOL (US DoD Committee, 1960) APL (Iverson, 1962) BASIC (Kemeny and Kurz, 1964) PL/I (IBM, 1964) SIMULA 67 (Nygaard and Dahl, 1967) ALGOL 68 (Committee, 1968) Pascal (Niklaus Wirth, 1971) C (Dennis Ritchie, 1972) Prolog (Alain Colmerauer, 1972) Smalltalk (Alan Kay, 1972) FP (Backus, 1978) Ada (UD DoD and Jean Ichbiah, 1983) C++ (Stroustrup, 1983) Modula-2 (Wirth, 1985) Delphi (Borland, 1988?) Modula-3 (Cardelli, 1989) ML (Robin Milner, 1985?) Eiffel (Bertrand Meyer, 1992) Java (Sun and James Gosling, 1993?) C# (Microsoft, 2001?) Scripting languages such as Perl, etc. Etc. Some Historical Perspective

20. Syntax and Semantics • Syntax is the set of rules that specify the composition of programs from letters, digits and other characters. • Semantics is the set of rules that specify what the result/outcome of a program is. • Problems with English language description of Syntax and Semantics: • verbosity • ambiguity

21. Syntax • What is syntax? • syntax vs. lexical rules • Regular languages and context-free languages • Backus Normal Form (a.k.a. Backus-Naur Form)BNF • A syntax metalanguage • Derivation vs. recognition • Syntax Diagram • Extended BNF (EBNF)

22. BNF History In Java: <IfThenStat> ::= if (<Expr>) <Stat> <IfThenElseStat> ::= if (<Expr>) <StatNoShortIf> else <Stat>

23. Recursive Descent Parsing • Parsing is the process of constructing a parse tree • A recursive descent parser is a kind of leftmost parser with very limited lookahead • Recursive descent parsers are built directly from (E)BNF rules • Recursive descent parsers do not work with left-recursive grammars • We provide a simple example for parsing terms made of factors

24. The Concept of Binding • entities (e.g., variables, statements, subprograms, declarations...) have attributes (e.g., for variable: name, type, storage area) • Binding is the specification of the exact nature of an attribute. • When does binding occur? Binding time. • language definition time • language implementation time • compile time • run time • Example: the Fortran type INTEGER is bound partly at language definition time, partly at language implementation time. • static (established before run-time, cannot be changed) and dynamic binding

25. Variables • Name • Scope • Lifetime • Value • l-value (memory location) and r-value (contents of a memory location) • Type

26. Variable Scope • Variables have scope: the range of program instructions over which the variable is known, and therefore manipulable • scope binding can be static or dynamic • dynamic scoping is easy to implement, but more confusing for most programmers • most modern languages use static scoping

27. Variable Type • type is the range of values a variable can take, together with operations to create, access, and modify values • variable type declarations • implicit in FORTRAN, explicit almost in any other language • dynamic binding between variables and types is unusual • APL and SNOBOL4 have it

28. Variable Value • binding of variable and value is dynamic, except for symbolic constants • Algol v. Pascal: manifest constants • reference (pointer) • access path (chain of pointers) • primary means of accessing anonymous variables • shared objects

29. Semantics • (So-called) Static Semantics • Context-Sensitive Grammars • Scope and Type • Attribute Grammars • Operational Semantics • Denotational Semantics • Axiomatic Semantics

30. Previewing Postscript In this course, most notes from the instructor are in Postscript format • Postscript previewers are installed on the departmental Unix machines • “gv is available on all solaris machines” (P. O’Keefe) • and on the College Windows machines • “GSView is located in all of the prototype machines. I copied the shortcuts in cse apps folder” (H. Naik) • a link to obtaining PostScript previewers is provided on the course web site. It points (indirectly) to http://www.cs.wisc.edu/~ghost/

31. Static Semantics • Static semantics are used to check type and scope rules • Semantics is a misnomer in this context • We provide only an example • for checking the type rules of the variable and expression in the assignment statement of a simple language • using attribute grammars (which are a formalization of contextual grammars)